1. Field of the Disclosure
The invention relates to a slip catalyst for reducing a NO2 content in an exhaust train of an internal combustion engine flowed through by an exhaust gas flow, in particular an exhaust train of a diesel engine, which has an oxidizing catalyst for the formation of NO2 and a particulate filter arranged downstream for binding carbon black particles and simultaneous and/or subsequent reaction of the same with NO2 formed on the oxidizing catalyst, comprising a substrate that is provided with a coating, which coating reduces a proportion of NO2 in the exhaust gas flow flowing through.
Furthermore, the invention relates to an exhaust train of an internal combustion engine, comprising an exhaust gas purification device flowed through by an exhaust gas flow, which has an oxidizing catalyst and optionally a particulate filter, wherein a slip catalyst for the reduction of a NO2 content in the exhaust train flowed through by the exhaust gas flow is arranged downstream.
2. Background Description
According to the prior art, particulate filters are provided in exhaust trains of diesel engines, which particulate filters filter out the carbon black particles contained in the diesel exhaust gas. However, with increasing load by carbon black particles, the particulate filters lead to a higher exhaust backpressure. In order to avoid this, it is necessary to incinerate continuously or intermittently at certain time intervals the carbon black particles located on a particulate filter or filtered out thereby. An increased exhaust backpressure and/or a drop in the engine power can thereby be avoided.
A corresponding concept lies in connecting an oxidizing catalyst upstream of a particulate filter, which oxidizing catalyst oxidizes the nitric oxide (NO) contained in the exhaust gas to nitrogen dioxide (NO2). The oxidizing catalyst is generally coated with a precious metal such as platinum or palladium for this purpose. In the particulate filter connected downstream, however, the carbon black particles are filtered out of the exhaust gas. The NO2 present in the exhaust gas and the NO2 (secondary NO2) formed on the oxidizing catalyst reacts on the particulate filter with the carbon black particles held there with incineration of the same. Systems of this type operate automatically from temperatures of 250° C., i.e. the particulate filter is constantly regenerated without additional measures being necessary. In this context it is called passive regeneration. A corresponding device is referred to as a Continuous Regenerating Trap (CRT). It is also possible in this connection to provide only one component, which acts as an oxidizing catalyst, but at the same time also filters carbon black particles out of the exhaust gas flow, which are immediately reacted with NO2 on the oxidizing catalyst.
The problem occurs in CRT systems that in all operating states, i.e., even at low operating temperatures in the exhaust train, an NO2 excess is desired on or in the particulate filter so that a passive regeneration of the particulate filter can take place or the carbon black particles are 100% converted. This can mean that a NO2 content at the end of the CRT system is increased compared to the exhaust gas directly exiting from the engine. An increased NO2 content in the discharged exhaust gas is not desired, however. In fact, legislation currently stipulates that an increase in the NO2 concentration in the exhaust train is no longer permissible or is permissible only within certain limits.
According to the prior art, it is possible with CRT systems to reduce a NO2 content in the exhaust gas flow by providing so-called Selective Catalytic Reduction (SCR). A solution containing urea is thereby injected into the exhaust train after the particulate filter. The solution containing urea releases ammonia and reacts at a downstream catalyst with NO2 with the formation of non-toxic gases. However, an SCR system requires a high operating expenditure. Thus, among other things, a tank must be provided for urea solution, which requires an additional expenditure in the production and operation.